Sensors for the pressure measurement in hot processes, which are used for example in a measuring borehole in the combustion chamber of an internal combustion machine, are subjected to extreme heat flows and pressure pulses. These heat flows lead to very high temperatures in the sensor, which may have an adverse effect on the measuring properties or lead to the destruction of the sensor. Measures are, therefore, known to protect the sensitive front section of the sensor, especially the sensor membrane, with the measuring behavior of the sensor remaining undisturbed by the protection device, however.
The membrane protection before the membrane of a sensor is used to reduce the heat input by thermal radiation and convection into the sensor. In the case of a spark-ignition engine, the flame front in the protection device is to be quenched by the thermal protection in order to reduce reactions of the sensor to the combustion. Protection shall also be ensured in the case of surface ignition and combustion knock in order to prevent the destruction of the sensor by the especially high heat input even under these extreme operating conditions.
In this connection, WO 2007/140641 A1 describes a membrane protection for a sensor which can be screwed into a borehole of a built-in component and is provided with a membrane in the front region, and which comprises a clip system for internal fastening in the front region of the sensor. A sensor with such a membrane protection can be mounted easily in a measuring borehole of a built-in component when the outside radius of the membrane protection is not larger than the outside radius of the sensor. The membrane protection, which comprises a plurality of small boreholes in order to produce contact with a medium to be measured, is provided with hook-like elements which engage into an internal groove in the end region of the sensor. The boreholes shall be provided with a very small diameter on the one hand in order to ensure sufficient membrane protection. Small borehole diameters have a rapid tendency towards coking by combustion residues on the other hand, as a result of which the measuring results will be impaired.
In this connection, a pressure sensor with membrane protection is known from U.S. Patent Publication No. 2004/0237629 A1, with embodiments of the membrane protection being provided with boreholes and oval and slit-like openings. The slit-like openings have a triangular cross-section and are arranged in a cross-like or star-like manner. In the latter case, the opening cross-sections of the membrane protection which are required for a dynamic pressure measurement are hardly achieved, with the regions of the triangular opening cross-sections which taper into a tip also having a tendency towards soot deposits and coking.
A pressure sensor is known from AT 002 036 U1, which comprises a screen-like thermal protection element which is arranged at a small distance from a coated sensor membrane. The thermal protection element comprises a plurality of boreholes parallel to the sensor axis. According to one embodiment, the thermal protection element can also have slit-like openings which are also arranged in parallel or radially.
Generally, thermal protection elements according to the state of the art face two opposing demands and therefore inevitably lead to an unsatisfactory compromise.
Demand A: Flame quenching shall also be enabled in highly charged spark-ignition engines with surface ignition and subsequent combustion knocking This demand can only be fulfilled by especially narrow slits or small boreholes of ≦0.2 mm, because there is a connection between pressure, pressure gradient and borehole cross section for secure quenching of the flame front. In the case of naturally aspirated engines with regular combustion, the diameter of the borehole could also be larger (e.g. 0.4 mm).
Demand B: The tendency towards coking shall be minimized. Modern direct-injection spark-ignition engines produce soot as a result of local lack of air, which can deposit on the walls of the small boreholes and in the cavity between the thermal protection element and sensor membrane, and can prevent precise pressure measurement. This tendency towards coking occurs especially in low borehole diameters and is therefore in conflict with demand A.
Notice must further be taken that for achieving a sufficiently large free cross-section of the thermal protection element at a borehole diameter of 0.2 mm approximately 100 boreholes are necessary even in the case of small sensors with a free cross-section of 7 mm2 (which corresponds to a diameter of approximately 3 mm), which leads to a very complex production effort.